There have been known a relay contact operated by a binary signal of ON/OFF or an ON/OFF means composed of semiconductor material used as external control means to control plants and equipment. In such an external control means, a signal sending side (hereafter referred to as the instruction side) from which signals are sent to the plant and equipment (hereafter referred to as the plant side) in which measurement, drive, or control is performed upon receiving the signals, is insulated from the plant side depending on use for the purpose of preventing affections of the signal voltage on the human body or evading affections of noise. This is generally done by adopting a photocoupler, signal insulation relay, insulation amplifier, and insulation transformer to send signals from the instruction side and to send measurement result from the plant side to the instruction side, and adopting an insulation transformer as a power source transformer to supply electric power.
In recent years, there has been an increased demand for performing soundness diagnosis of circuits in the field of instrumentation and measurement for the purpose of raising reliability of the system by confirming soundness of output signals and circuit wiring, that is, by confirming whether the measurement and control means are operating correctly as instructed and whether there is a breaking of wire or short circuit occurred in the circuit.
FIG. 8 and FIG. 9 respectively shows an example of conventional drive circuit of an external control means such as a relay contact operated by a binal signal of ON/OF or ON/OFF means composed of semiconductor material, and that added with a circuit for diagnosing soundness of the drive circuit.
In FIG. 8 and FIG. 9, binary signals 153 and 154 for driving and controlling external control means 151 and 152 (hereafter referred to as contacts depending on circumstances) are inputted respectively to photoMOS relays 155 and 156, for example, used for insulating the instruction side from the plant side, and sent to the contacts 151 and 152 to drive them. A broken line 157 indicates an insulation barrier to insulate the plant side from the instruction side by the photoMOS relays 155 and 156. The signals sent to the contacts 151 and 152 are also insulated by the insulation barrier.
In the circuit of FIG. 8, the binary signals 153 and 154 to switch on/off the contacts 151 and 152 as external control means are merely sent to the photoMOS relays 155 and 156, whether the photoMOS relays 155, 156, contacts 151, 152 are operating normally, or whether there is a breaking of wire or short circuit occurred, can not be detected.
A dedicated circuit is required in order to perform soundness diagnosis of the circuit such as to know whether the photoMOS relays 155, 156, contacts 151, 152 are operating normally, and whether a breaking of wire or short circuit has not occurred.
An example when a dedicated circuit is provided is shown in FIG. 9 in a block diagram. In FIG. 9 are provided electric power source circuits 101a and 101b which are not provided in the circuit of FIG. 8. A diagnosing circuit comprised of a microcomputer is needed to diagnose operation status of each of the contacts, and the electric power source circuits 101a and 101b are necessary respectively to supply electric power to drive each of the diagnosing circuits. Each of the electric power source circuits 101a, 101b comprises an electric power source 102a, 102b, pulse generating circuit 103a, 103b, insulation transformer 104a, 104b, rectifying circuit 105a, 105b, and constant voltage circuit 106a, 106b. 
The binary signal 153, 154 for switching on/off the contact 151, 152 is inputted to the photocoupler 155, 156 used for the purpose of insulating the plant side from the instruction side similarly to the case of FIG. 8, then converted into an ON/OFF signal to switch on/off the contact by a monitoring circuit 158a, 158b for monitoring the contact signal and condition of the contact for reading over, the monitoring circuit being supplied with electric power from the electric power source circuit 101a and 101b, to be sent to the contact 151, 152 to drive each contact.
Operation status such as whether the contact 151, 152 is operating as instructed and whether there is a breaking of wire or short circuit occurred is diagnosed by the monitoring circuit 158a, 158b and outputted from the photocoupler 159a, 159b as a read over signal sent from the plant side to the instruction side.
However, in the circuit of FIG. 9, the insulation electric power source circuit 101a, 101b, and monitoring circuit 158a, 158b for monitoring operation status of the driving signal for driving the external control means and the external control means are provided for each contact 151 and 152 in the plant side, so, manufacturing cost increases very much and actual example of use is limited to very special use.
That is the conventional circuit to perform driving of circuit and concurrently diagnosis of circuit soundness whether the signal for switching on/off the external control means such as a relay contact actuated by a binary signal of ON/OFF or ON/OFF means composed of semiconductor material used for the control of a plant or equipment is correctly transmitted to the external control means, whether the external control means is operating normally, and whether there is a breaking of wire or short circuit occurred. With this conventional circuit construction for diagnosing operation status of the external control means, there have been problems as follows:
(A) An electric power source circuit and soundness diagnosing circuit must be added to perform soundness diagnosis separately from the transmitting circuit of ON/OFF signal to the contact, so, the number of parts increases and circuit construction becomes complicated, resulting in increased manufacturing cost.
(B) When there are a plurality of contacts and the contacts must be insulated from one another as shown in FIG. 9, a soundness diagnosis circuit must be provided for each of the contacts, and when the contacts are located distantly from one another, an expensive insulation electric power source circuit must be provided for each of the contacts, so, manufacturing cost increases further.
(C) To cope with this, when a single expensive insulation power source is provided for common use for each of the contacts,
and when the contacts are located distantly from one another, difference in voltage occurs due to difference in voltage drop caused by difference of length of signal wires. Therefore, it has been necessary principally to perform diagnosis of contacts located near to one another.
As to the art for detect braking of wire, there are disclosed for example in patent literature 1(Japanese Laid-Open Patent Application No. 2006-023105) a method of detecting breaking of wire by applying a pulse signal to the wire, and comparing the current wave shape measured with the reference current wave shape to judge the presence or absence of breaking of wire from difference in both the wave shapes, and in patent literature 2 (Japanese Laid-Open Patent Application No. 2004-198302) a circuit for detecting breaking of wire by applying a pulse signal for checking via an impedance component to the signal wire for detecting breaking of wire, and comparing the signal obtained from the signal wire with the pulse signal for checking to judge the presence or absence of breaking of wire.
As to diagnosis of electric circuits, there is disclosed for example in patent literature 3(Japanese Laid-Open Patent Application No. 8-005708) a method of diagnosing electric circuits and diagnosing device used for the method. With the method, conditions of electric apparatuses are diagnosed for the purpose of improving efficiency of diagnosis operation by facilitating measurement record management and further decreasing occurrence of man-caused errors, by reading out information written and stored in a nonvolatile memory concerning measurement results of characteristics or things concerning measurement of the electric apparatuses, or measurement results of characteristics or things concerning measurement of the electric apparatuses, and comparing the read-out information with the information of-the-moment concerning measurement results of characteristics or things concerning measurement of the electric apparatuses.
However, with the art taught in the patent literature 1 and 2, means for applying pulse signals and a memory for memorizing reference current wave shape are needed, and with the electric circuit diagnosing device disclosed in the patent literature 3, a memory memorized information concerning measurement results of characteristics or things concerning measurement of the electric apparatuses is needed, and further a means for measuring characteristics of the circuit and a means for comparing the measurement result with the reference data, resulting in complicated composition. Therefore, problems cited in the items (A)˜(C) cannot be solved by these art.